Nonaqueous electrolyte secondary battery having a negative electrode containing carbon fibers and carbon flakes

ABSTRACT

A nonaqueous electrolyte secondary battery is provided with a positive electrode including a positive-electrode active material, a negative electrode including a negative-electrode active material, and a nonaqueous electrolyte solution. The negative electrode further includes carbon fibers and carbon flakes. The synergistic effects of the improved retention of the electrolyte solution by the carbon fibers and the improved conductivity between the active material particles by the carbon flakes facilitate doping/undoping of lithium in a high-load current mode and increase the capacity of the battery in the high-load current mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation of co-pending U.S. application Ser. No.09/675,422, filed Sep. 29, 2000, which claims priority from JapaneseApplication No. P11-278249, filed Sep. 30, 1999. Applicant claimspriority to and the benefit of the above-identified applications, thedisclosures of which are expressly incorporated herein by reference tothe extent permitted by law.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to nonaqueous electrolyte secondarybatteries having positive-electrode active materials andnegative-electrode active materials, which intercalate/deintercalate (orare doped/undoped with) lithium, and nonaqueous electrolyte solutions.

[0003] Nickel-cadmium batteries and lead batteries have been used assecondary batteries for electronic devices. Trends toward higherperformance and miniaturization of electronic devices due to advancedelectronic technology require secondary batteries having higher energydensities. Since nickel-cadmium batteries and lead batteries have lowdischarge voltages, increases in the energy densities are limited.

[0004] Nonaqueous electrolyte secondary batteries using carbonaceousmaterials capable of intercalating/deintercalating lithium in negativeelectrodes and lithium compound oxides in positive electrodes have beenvigorously developed instead of the nickel-cadmium batteries and leadbatteries, since the nonaqueous electrolyte secondary batteries, calledlithium ion batteries, have high discharge voltages and reducedself-discharge, and have prolonged cycle lives.

[0005] In these nonaqueous electrolyte secondary batteries, carbonaceousmaterials such as graphite are used as negative-electrode activematerials, Li_(x)MO₂ wherein M is at least one transition metal and0.05<x<1.10 is used as positive-electrode active materials, and LiPF₆and LiBF₄ are used as electrolytes. As organic solvents for dissolvingthe electrolytes, propylene carbonate, ethylene carbonate,γ-butyrolactone, diethyl carbonate, ethyl methyl carbonate, dimethylcarbonate, ethyl acetate, methyl propionate, 1,2-dimethoxyethane, and2-methyltetrahydrofuran are used.

[0006] The nonaqueous electrolyte secondary batteries are suitable aspower sources for portable electronic devices. In recent years, compactbattery packs including batteries and protective circuits have beenfrequently used with requirements for reduced sizes and weight. In thebatteries in the battery packs, higher capacities at large-currentdischarging modes are required. In order to fulfill such a requirement,improvements in negative electrodes are essential in addition toimprovements in positive electrodes and nonaqueous electrolytesolutions.

[0007] Current nonaqueous electrolyte secondary batteries, however, arestill unsatisfactory as regards improvements in capacities duringlarge-current discharging modes by improvements in negative electrodes.

BRIEF SUMMARY OF THE INVENTION

[0008] Accordingly, it is an object of the present invention to providea nonaqueous electrolyte secondary battery having satisfactory capacitycharacteristics during a large-current discharging mode.

[0009] According to an aspect of the present invention, a nonaqueouselectrolyte secondary battery comprises a positive electrode comprisinga positive-electrode active material, a negative electrode comprising anegative-electrode active material, the positive-electrode activematerial and the negative-electrode active material capable ofintercalating/deintercalating lithium, and a nonaqueous electrolytesolution, wherein the negative electrode further comprises carbon fibersand carbon flakes.

[0010] The carbon fibers and carbon flakes can be disposed in theinterstices between the negative-electrode active material particles inthis configuration. Moreover, the carbon fibers improves retention ofthe nonaqueous electrolyte solution and the carbon flakes disposedbetween the active material particles improves conductivity (reducesinternal resistance). These synergistic effects improve the capacitycharacteristics of the nonaqueous electrolyte secondary battery. In thisnonaqueous electrolyte secondary battery, lithium is smoothly doped orundoped at large-current charge or discharge conditions, resulting inhigh capacity at the high-current load.

[0011] In this nonaqueous electrolyte secondary battery, the content ofthe carbon fibers in the negative electrode is in a range of preferably0.02 percent by weight to 5 percent by weight and more preferably 0.5percent by weight to 4 percent by weight, and the content of the carbonflakes in the negative electrode is in a range of preferably 0.1 percentby weight to 30 percent by weight, more preferably 1 percent by weightto 20 percent by weight, most preferably 1 percent by weight to 10percent by weight.

[0012] Moreover, the ratio by weight of the carbon fibers to the carbonflakes in the negative electrode is in a range of preferably 0.2 to 100and more preferably 0.4 to 20.

[0013] In a preferred embodiment of the present invention, thenonaqueous electrolyte secondary battery comprises an electrodecomposite in which a positive electrode including a lithium compoundoxide as the positive-electrode active material and a negative electrodeincluding a carbonaceous material as the negative-electrode activematerial are wound with a separator disposed therebetween (called ajelly roll type).

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0014]FIG. 1 is an enlarged cross-sectional view of a negative electrodein accordance with an embodiment of the present invention;

[0015]FIG. 2 is a graph showing the relationships between the capacityat a constant current of 5 A and the content of carbon fibers VGCF in anegative electrode and between the internal resistance at 23° C. and thecarbon fiber content;

[0016]FIG. 3 is a graph showing the relationships between the capacityat a constant current of 5 A and the content of carbon flakes KS-15 in anegative electrode and between the internal resistance at 23° C. and thecarbon flake content;

[0017]FIG. 4 is a graph showing the relationships between the capacityat a constant current of 5 A and the ratio by weight of carbon flakes tocarbon fibers in a negative electrode and between the internalresistance at 23° C. and the ratio; and

[0018]FIG. 5 is a longitudinal cross-sectional view of a nonaqueouselectrolyte secondary battery in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] Preferred embodiments of the present invention will now bedescribed in detail with reference to the drawings.

[0020]FIG. 5 is a longitudinal cross-sectional view of a nonaqueouselectrolyte secondary battery in accordance with an embodiment of thepresent invention. This secondary battery includes an electrodecomposite 11 between a cylindrical casing 1 and a mandrel 2, in which astack of a collector 3 and a negative electrode 5 and another stack of acollector 4 and a positive electrode 6 are alternately wound with aseparator 7 provided therebetween. The electrode composite 11 contains anonaqueous electrolyte solution (not shown in the drawing). Insulatingplates 8 are provided on and under the electrode composite 11. Theelectrode composite 11 is covered by the bottom 10 of the battery casingat the bottom section and by a safety valve 13 and a lid 14 at the topsection. The bottom 10 of the battery casing is connected to a negativelead 9, whereas the lid 14 is connected to a positive lead 12. Thesafety valve 13 is provided to release pressurized internal gasgenerated by charging/discharging cycles. A gasket 15 insulates thepositive electrode and the negative electrode from each other. Thesecondary battery further has a positive temperature coefficient (PTC)element 16 to prevent overdischarge and overcharge currents.

[0021] Both the negative electrode 5 and the positive electrode 6 canintercalate/deintercalate (can be doped/undoped with) lithium in thisnonaqueous electrolyte secondary battery. The negative electrode 5 andthe positive electrode 6 are separated by the separator 7 and areimmersed in the nonaqueous electrolyte solution of an organic solventand a lithium compound.

[0022] The positive electrode 6 comprises a positive-electrode activematerial such as a lithium compound oxide, whereas the negativeelectrode 5 comprises a negative-electrode active material, such asgraphite. The negative electrode 5 further comprises carbon fibers andcarbon flakes. These active materials are held on the collectors 3 or 4composed of a metal foil or the like and are used as electrodes. Boththe positive-electrode active material and the negative-electrode activematerial have layered molecular structures, which can intercalate anddeintercalate lithium.

[0023] These electrode materials do not substantially react with thenonaqueous electrolyte solution and lithium and migrates in thenonaqueous electrolyte solution. In a discharging mode, lithium isdetached from the negative electrode 5, passes through the separator 7,and is intercalated in the positive electrode 6. In a charging mode,lithium is detached from the positive electrode 6, passes through theseparator 7, and is intercalated in the negative electrode 5.

[0024] In this embodiment, as shown in FIG. 1, the negative electrode 5comprises particles 17 of a negative-electrode active material which arebonded to each other with a binder (not shown in the drawing) on thecollector 20 composed of, for example, copper. The negative electrode 5further includes carbon fibers 18. Since the carbon fibers 18 are thinand long compared to the particles 17, these are disposed in theinterstices between the negative-electrode active material particles 17.Since the nonaqueous electrolyte solution can be immersed in the overallinterstices between the negative-electrode active material particles 17,this configuration can improve the retention of the nonaqueouselectrolyte solution. In the present invention, carbon flakes 19 arealso included in the negative-electrode active material particles 17.Since the carbon flakes 19 are flat and have high electron conductivitydue to high crystallinity, the flakes enter the interstices between thenegative-electrode active material particles 17 and improve the contactbetween these particles 17, resulting in improvement in conductivitybetween the negative- electrode active material particles 17.

[0025] In order to secure the above effects, the content of the carbonfibers in the negative electrode is in a range of preferably 0.02percent by weight to 5 percent by weight and more preferably 0.5 percentby weight to 4 percent by weight, and the content of the carbon flakesin the negative electrode is in a range of preferably 0.1 percent byweight to 30 percent by weight, more preferably 1 percent by weight to20 percent by weight, most preferably 1 percent by weight to 10 percentby weight. Moreover, the ratio by weight of the carbon fibers to thecarbon flakes in the negative electrode is in a range of preferably 0.2to 100 and more preferably 0.4 to 20. The content of thenegative-electrode active material particles 17 is preferably in a rangeof 65 to 99.88 percent by weight.

[0026] Preferably, the carbon fibers have an average diameter of 0.01 to1 μm and an average length of 1 to 100 μm, and the carbon flakes have anaverage diameter of 0.5 to 50 μm and an average thickness of 0.01 to 1μm. The negative-electrode active material particles 17 preferably havean average diameter of 1 to 100 μm.

[0027] The materials constituting the positive electrode 6 will now bedescribed in more detail.

[0028] As the negative-electrode active materials which can intercalateand deintercalate lithium, for example, carbonaceous materials can beused. Examples of carbonaceous materials include pyrolyzed carbon, cokessuch as pitch coke, needle coke, and petroleum coke, graphite, glassycarbon, baked organic polymers, such as phenol resin and furan resin,carbon fibers, and activated charcoal.

[0029] Other usable materials for the negative electrode are, forexample, crystalline and amorphous metal oxides which canintercalate/deintercalate lithium. Among these materials, graphite, softcarbon (graphitizable carbon), and hard carbon (nongraphitizable carbon)are preferable. It is preferable that the material for the negativeelectrode contain a certain amount of resin component.

[0030] The graphite may be natural graphite or artificial graphite. Apreferable graphite has a d₀₀₂ distance (a degree of graphitization) ofapproximately 0.336 nm, an L_(c) value (the thickness of the layer inthe c axis) of more than 100, a D₅₀ particle size of approximately 30nm, and a BET value (an index for the specific surface area) ofapproximately 2 m²/g. A preferable hard carbon has a d₀₀₂ distance ofapproximately 0.37 to 0.38 nm.

[0031] This negative-electrode active material, which canintercalate/deintercalate lithium, is mixed with the carbon fibers andthe carbon flakes to form the negative electrode.

[0032] The carbon fibers can be provided by a heat treatment of aprecursor which is composed of fibrous polymer or pitch, or by vapordeposition in which a stream of a vaporized organic material, such asbenzene, is directly exposed to a substrate at a temperature ofapproximately 1,000° C. so that carbon crystals grow in the presence ofiron particles as a catalyst.

[0033] Polymer precursors when carbon fibers are provided by the heattreatment are polyacrylonitrile (PAN) and rayon. Polyamide, lignin, andpolyvinyl alcohol are also usable.

[0034] Examples of pitch-based precursors include coal tar, ethylenebottom products, tars which are produced by high-temperature cracking ofcrude oil, products from asphalt by distillation, such as vacuumdistillation, atmospheric distillation, or steam distillation, thermalcondensation, extraction, or chemical condensation, and pitch formedduring carbonization of wood.

[0035] Also, examples of starting materials for the pitch includepolyvinyl chloride resin, polyvinyl acetate, polyvinyl butyral, and3,5-dimethylphenol.

[0036] In the carbonization process, the pitch from charcoal is presentas liquid at a temperature of up to about 400° C., and aromatic ringsare accumulated and oriented by condensation and polycyclization at thetemperature. The accumulated aromatic rings are converted to a solidprecursor, that is, semicoke, at a temperature of 500° C. or more. Thisprocess is called a liquid-phase carbonization process which is atypical process for graphitizable carbon.

[0037] Examples of usable raw materials for the pitch include fusedpolycyclic hydrocarbons, such as naphthalene, phenanthrene, anthracene,triphenylene, pyrene, perylene, pentaphene, and pentacene; derivativesthereof, such as carboxylic acids, carboxylic anhydrides, and carboxylicimides; mixtures thereof; fused heterocyclic compounds, such asacenaphthylene, indole, isoindole, quinoline, isoquinoline, quinoxaline,phthalazine, carbazole, acridine, phenazine, and phenanthrazine; andderivatives thereof.

[0038] Both polymer-based precursors and pitch-based precursors aresubjected to infusibilization or stabilization and then a heat treatmentat high temperatures to form carbon fibers. In the infusibilization, thesurface of the precursor is oxidized with oxygen or ozone so that theprecursor is not fused nor pyrolyzed during carbonization. Although theprocess can be appropriately determined depending on the type of theprecursor, the infusibilization temperature must be lower than themelting point of the precursor. The infusibilization may be repeatedseveral times, if necessary, to sufficiently stabilize the precursor.

[0039] The infusibilized or stabilized precursor is carbonized in anitrogen or inert gas atmosphere at 300 to 700° C. Next, the precursoris calcined in an inert gas atmosphere at a heating rate of 1 to 100°C./min and then at a constant temperature of 900 to 1,500° C. for 0 to30 hours form the carbon fibers. The carbonization can be omitted insome cases.

[0040] When the carbon fibers are produced by vapor phase deposition,any vaporizable organic compound can be used as a starting material.Examples of such materials include vaporizable materials at roomtemperatures, such as benzene, ethylene, and propane, and organiccompounds which are vaporizable at temperatures less than pyrolytictemperatures thereof. The vaporized organic compound is directly exposedto a hot substrate so that fibrous carbon crystals grow. The temperatureis preferably in a range of 400° C. to 1,500° C. and depends on the typeof the organic starting material. The substrate is preferably selectedfrom quartz and nickel, and depends on the type of the organic startingmaterial.

[0041] Any catalyst may used to promote the crystal growth. Examples ofusable catalysts are particles of iron, nickel, and a mixture thereof.Also, metals and oxides thereof, which are called graphitizingcatalysts, are used. The diameter and the length of the carbon fiberscan be appropriately determined by the production conditions.

[0042] When the polymer is used as the raw material, the diameter andthe length can be appropriately determined by the inner diameter of anozzle and a drawing rate from the nozzle when the fibers are produced.When the vapor deposition process is used, the sizes of the substrateand the catalyst which function as nuclei for the crystal growth areappropriately selected to determine the optimum diameter of the fibers.The feeding rate of the organic compound, such as ethylene or propane,determines the diameter and the linearity of the fibers.

[0043] The carbon fibers may be graphitized in an inert gas atmosphereat a heating rate of 1 to 100° C./min and then at a constant temperatureof 2,000° C. or more (preferably 2,500° C. or more) for 0 to 30 hours.The resulting carbon fibers may be pulverized depending on the thicknessof the electrode and the particle size of the active material. Filamentsproduced during spinning are also usable. The pulverization may beperformed before or after the carbonization or calcination, or in theheating step before the graphitizing.

[0044] The carbon flakes may be natural graphite or artificial graphitewhich are formed by carbonization and heat-treating an organic materialsuch as coal or pitch.

[0045] The natural graphite is quarried in China, Madagascar, Sri Lanka,Mexico, and Brazil. The graphite ores contain many organic impurities.In particular, metal elements are electrochemically dissolved and willadversely affect the performance of the battery. Thus, the impuritiesare preferably removed using a solvent. Examples of such solventsinclude an aqueous inorganic acid solution containing hydrogen fluorideor hydrogen chloride, an organic acid solution, an aqueous alkalinesolution containing sodium hydroxide, an aqueous alkaline organicsolution, and an organic solvent.

[0046] Examples of pitches include coal tar, ethylene bottom products,tars which are produced by high-temperature cracking of crude oil,products from asphalt by distillation, such as vacuum distillation,atmospheric distillation, or steam distillation, thermal condensation,extraction, or chemical condensation, and pitch formed duringcarbonization of wood.

[0047] Also, examples of starting materials for the pitch includepolyvinyl chloride resin, polyvinyl acetate, polyvinyl butyral, and3,5-dimethylphenol.

[0048] Examples of usable raw materials for the pitch include fusedpolycyclic hydrocarbons, such as naphthalene, phenanthrene, anthracene,triphenylene, pyrene, perylene, pentaphene, and pentacene; derivativesthereof, such as carboxylic acids, carboxylic anhydrides, and carboxylicimides; mixtures thereof; fused heterocyclic compounds, such asacenaphthylene, indole, isoindole, quinoline, isoquinoline, quinoxaline,phthalazine, carbazole, acridine, phenazine, and phenanthrazine; andderivatives thereof.

[0049] In the formation of desired granular carbon from the aboveorganic material, for example, the organic material is carbonized in aninert gas atmosphere at 300 to 700° C. The carbonized material iscalcined in an inert gas atmosphere at a heating rate of 1 to 100°C./min and then at a holding temperature of 900 to 1,500° C. for 0 to 30hours to form a graphitizable carbonaceous material, and is heat-treatedat 2,000° C. or more and preferably 2,500° C. or more. The carbonizationand calcination may be omitted in some cases.

[0050] The natural or artificial graphite material is pulverized andclassified to prepare graphite flakes. The graphite material must havehigh crystallinity in order to obtain the graphite flakes. Flatterflakes are obtainable by cleaving between carbon hexagonal planes whichare weakly bonded by van der Waals forces. The high degree ofcrystallinity is required for achieving sufficient electron conductivityas a conductor.

[0051] The carbon flakes preferably have a (002) interplanar spacing ofless than 0.3360 nm by X-ray diffractometry and a thickness of (002)c-axis crystallites of 100 nm or more. Preferably, the bulk densitymeasured according to Japanese Industrial Standard (JIS) K-1469 is 0.4g/cm3 or less, and the maximum particle size by laser diffraction is 50μm or less.

[0052] The positive electrode material is not limited in the presentinvention. It is preferable that the positive electrode contain asufficient amount of lithium. Preferable positive electrode materialsare metal compound oxides represented by the general formulaLiM_(x)O_(y) comprising lithium and transition metals wherein M is atleast one selected from Co, Ni, Mn, Fe, Al, V, and Ti; and intercalationcompounds containing lithium.

[0053] Examples of the binder for binding the negative-electrodematerials and the positive electrode materials are polyvinylidenefluoride, polytetrafluoroethylene, an ethylene-propylene-dienecopolymer, a styrene-butadiene rubber, a polyimide, a polyamide-imide,polyvinyl alcohol, and carboxymethyl cellulose.

[0054] Examples of the nonaqueous electrolytes include electrolytesolutions of electrolytes dissolved in nonaqueous solvents, solidelectrolyte media of electrolytes in polymers, and gelatinouselectrolyte solutions of plasticizers and electrolytes dissolved inpolymers. Examples of polymers include silicones, polyacrylates,polyacrylonitrile, and polyethylene oxide; mixtures, crosslinkedpolymers, and modified polymers thereof; and fluorinated polymers, suchas polyvinylidene fluoride, polyhexafluroropropylene,polytrifluoroethylene, copolymers thereof, and mixtures thereof. As theplasticizers, nonaqueous solvents and organic solvents are usable.

[0055] Also, known solutions of electrolytes in organic solvents areusable. Examples of the organic solvents include propylene carbonate,ethylene carbonate, vinylene carbonate, dimethyl carbonate, diethylcarbonate, methyl ethyl carbonate, 1,2-dimethoxyethane,1,2-diethoxyethane, (-butyrolactone, tetrahydrofuran, 1,3-dioxolane,4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane,acetonitrile, and propionitrile. Examples of the electrolytes includeLiClO₄, LiAsF₆, LiPF₆, LiBF₄, LiB(C₆H₅)₄, LiCi, LiBr, CH₃CO₂Li,CF₃SO₃Li, and (CnF_(2n+1)SO₃)₂NLi.

[0056] The nonaqueous electrolyte secondary battery in accordance withthe present invention may have any shape according to use. Examples ofthe shapes are cylindrical shown in FIG. 5, buttons, prisms, and coins.

[0057] As described above, in the nonaqueous electrolyte secondarybattery in accordance with the present invention, the carbon fibers andthe carbon flakes are disposed in the interstices between the negativematerial particles. The carbon fibers improve the retention of theelectrolyte solution while the carbon flakes improve conductivitybetween the active material particles. These synergistic effects reducethe internal resistance in the battery and facilitate doping/undoping oflithium under a large-current discharging condition (a high-load currentmode), resulting in improved capacity characteristics in a high-loadcurrent mode. The negative electrode composed of such a compositematerial has high packing density and has low internal resistance orhigh conductivity. As a result, the nonaqueous electrolyte secondarybattery of the present invention has high energy density.

EXAMPLES

[0058] Examples of the present invention will now be described.

Example 1

[0059] To 100 parts by weight of coal coke as a filler, 30 parts byweight of coal tar pitch as a binder was added. These were mixed atapproximately 100° C., and were compression-molded by a press to form aprecursor. The precursor was heat-treated at 1,000° C. or less to form acarbonaceous molded article. The carbonaceous molded article wassubjected to a pitch impregnation step for impregnating a binder pitchmelted at 200° C. or less therein and a calcination step for heating at1,000° C. or less. These steps were repeated several times. Thecarbonaceous molded article was heat-treated in an inert gas atmosphereat 2,700° C. to form a graphitized molded article. The graphitizedmolded article was pulverized and classified to preparenegative-electrode active material particles.

[0060] This graphite material had a d₀₀₂ distance of approximately 0.337nm, a thickness of the c-axis crystallites in the (002) plane of 50.0nm, a sphericity by pycnometry of 2.23, a bulk density of 0.83 g/cm³, anaverage shape parameter X_(ave) of 10, a BET specific area of 4.4 m²/g,an average particle diameter by laser diffractometry of 31.2 μm, an 10%accumulated particle size of 12.3 μm, a 50% accumulated particle size of29.5 μm, a 90% accumulated particle size of 53.7 μm, and an averagerupture strength of graphite particles of 7.1 kgf/mm².

[0061] The resulting powdered sample was mixed with 1 percent by weightof carbon fibers VGCF having an average diameter of 0.2 μm and anaverage length of 15 μm made by Showa Denko K. K. and 5 percent byweight of carbon flakes KS-15 having an average diameter of 9 μm and anaverage thickness of 0.1 μm made by Lonza A. G.

[0062] Using the mixture as a negative electrode material, a cylindricalnonaqueous electrolyte secondary battery shown in FIG. 5 was fabricated,as follows. A negative electrode composition was prepared by mixing 90parts by weight of the mixture and 10 parts by weight of polyvinylidenefluoride (PVDF) as a binder and was dispersed into N-methylpyrrolidoneto form a slurry. The slurry was applied on both faces of a collector 3of a copper foil strip having a thickness of 10 μm, was dried, and wascompressed under a predetermined pressure to form a negative electrodestrip 5.

[0063] A positive-electrode active material was prepared as follows. Amixture of 0.5 mole lithium carbonate and 1 mole cobalt carbonate wassintered in air at 900° C. for 5 hours. The X-ray diffraction pattern ofthe resulting material agreed with that of LiCoO₂ registered in theJCPDS (Joint Committee Powder Diffraction Standards) file.

[0064] The LiCoO₂ was pulverized. The LiCoO₂ powder had an 50%accumulated particle size by laser diffractometry of 15 μm. Next, 95parts by weight of LiCoO₂ powder and 5 parts by weight of lithiumcarbonate were mixed, and 91 parts by weight of mixture, 6 parts byweight of flake graphite as a conductor, and 3 parts by weight ofpolyvinylidene fluoride as a binder were mixed to prepare a positiveelectrode composition. The composition was dispersed intoN-methylpyrrolidone to form a slurry. The slurry was applied on bothfaces of a collector 4 of a copper foil strip having a thickness of 20μm, was dried, and was compressed under a predetermined pressure to forma positive electrode strip 6.

[0065] As shown in FIG. 5, a separator 7 formed of a microporouspolypropylene film having a thickness, the negative electrode 5, anotherseparator 7, and the positive electrode 6 were stacked and were woundseveral times to form a spiral electrode composite 11 which was able tobe contained in a casing having an outer diameter of 18 mm.

[0066] The spiral electrode composite 11 was contained in anickel-plated iron casing 1. Insulating plates 8 were placed on andunder the spiral electrode composite 11. An aluminum positive-electrodelead 12 extending from the positive-electrode collector 4 and a nickelnegative-electrode lead 9 extending from the negative-electrodecollector 3 were welded to a lid 4 and the casing 1, respectively.

[0067] An electrolyte solution in which an equivolume mixture ofethylene carbonate (EC) and dimethyl carbonate (DMC) was dissolved inLiPF₆ in a concentration of 1.0 mol/l was injected into the casing 1.The casing 1 was caulked using an insulating gasket 15 which was coatedwith asphalt to fix a safety valve 13, a PTC element 16, and a lid 14and to ensure hermetic sealing of the battery. A cylindrical nonaqueouselectrolyte secondary battery having a diameter of 18 mm and a height of65 mm was thereby prepared.

Example 2

[0068] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 3 percent by weight of VGCF and 5 percent by weight ofKS-15 were mixed therein.

Example 3

[0069] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 4 percent by weight of VGCF and 5 percent by weight ofKS-15 were mixed therein.

Example 4

[0070] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 4.8 percent by weight of VGCF and 5 percent by weight ofKS-15 were mixed therein.

Example 5

[0071] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 5 percent by weight of VGCF and 5 percent by weight ofKS-15 were mixed therein.

Example 6

[0072] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 0.5 percent by weight of VGCF and 5 percent by weight ofKS-15 were mixed therein.

Example 7

[0073] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 0.05 percent by weight of VGCF and 5 percent by weight ofKS-15 mixed therein.

Example 8

[0074] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 0.02 percent by weight of VGCF and 5 percent by weight ofKS-15 were mixed therein.

Example 9

[0075] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but I percent by weight of VGCF and 10 percent by weight ofKS-15 were mixed therein.

Example 10

[0076] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 1 percent by weight of VGCF and 20 percent by weight ofKS-15 were mixed therein.

Example 11

[0077] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but I percent by weight of VGCF and 28 percent by weight ofKS-15 were mixed therein.

Example 12

[0078] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 1 percent by weight of VGCF and 30 percent by weight ofKS-15 were mixed therein.

Example 13

[0079] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 1 percent by weight of VGCF and 1 percent by weight ofKS-15 were mixed therein.

Example 14

[0080] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 1 percent by weight of VGCF and 0.5 percent by weight ofKS-15 were mixed therein.

Example 15

[0081] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 1 percent by weight of VGCF and 0.2 percent by weight ofKS-15 were mixed therein.

Example 16

[0082] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 1 percent by weight of VGCF and 0.1 percent by weight ofKS-15 were mixed therein.

Example 17

[0083] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but the VGCF and the KS-15 were not mixed therein.

Example 18

[0084] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but only 1 percent by weight of VGCF was mixed therein.

Example 19

[0085] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but only 5 percent by weight of KS-15 was mixed therein.

Example 20

[0086] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 0.01 percent by weight of VGCF and 5 percent by weight ofKS-15 were mixed therein.

Example 21

[0087] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 6 percent by weight of VGCF and 5 percent by weight ofKS-15 were mixed therein.

Example 22

[0088] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 1 percent by weight of VGCF and 0.05 percent by weight ofKS-15 were mixed therein.

Example 23

[0089] A nonaqueous electrolyte secondary battery was prepared as inExample 1, but 1 percent by weight of VGCF and 40 percent by weight ofKS-15 were mixed therein.

Evaluation of Battery Characteristics

[0090] Each secondary battery was charged under a constant-voltage,constant-current condition of a constant current of 0.5 A and a maximumvoltage 4.2 V for 4 hours at 23° C. The secondary battery was dischargedat a constant current of 0.5 A at 23° C. until the final voltage reached2.75 V to determine the initial capacity. Next, the secondary batterywas charged under a constant-voltage, constant-current condition of aconstant current of 1.0 A and a maximum voltage 4.2 V for 2.5 hours at23° C. An AC impedance was measured at a frequency of 1 KHz and anapplied voltage of 10 mV to determine the internal resistance of thebattery. Next, the secondary battery was discharged at a constantcurrent of 5 A at 23° C. until the final voltage reached 2.75 V todetermine the battery capacity.

[0091] Table 1 shows the results. FIG. 2 shows the dependence of batterycharacteristics when the KS-15 content is fixed to 5 percent by weight,and FIG. 3 shows the dependence of battery characteristics on the KS-15content when the VGCF content is fixed to 1 percent by weight. FIG. 4shows the dependence of battery characteristics on the ratio by weightof the KS-15 to the VGCF. TABLE 1 Carbon Fiber Carbon Flake KS-15/Initial Capacity Internal VGCF content KS-15 content VGCF Capacity at 5Resistance at Example (% by weight) (% by weight) Ratio (mAh) A (mAh)25° C. (mΩ) 1 1 5 5.00 1,603 1,053 55 2 3 5 1.67 1,610 1,055 54 3 4 51.25 1,608 950 59 4 4.8 5 1.04 1,601 710 64 5 5 5 1.00 1,600 700 66 60.5 5 10.00 1,605 985 58 7 0.05 5 100.00 1,606 698 66 8 0.02 5 250.001,602 605 70 9 1 10 10.00 1,603 880 61 10 1 20 20.00 1,602 680 67 11 128 28.00 1,600 609 69 12 1 30 30.00 1,599 600 70 13 1 1 1.00 1,605 78562 14 1 0.5 0.50 1,608 751 62 15 1 0.2 0.20 1,610 620 69 16 1 0.1 0.101,615 601 70 17 0 0 — 1,600 481 83 18 1 0 0.00 1,625 510 75 19 0 5 —1,602 509 75 20 0.01 5 500.00 1,603 511 74 21 6 5 0.83 1,590 510 75 22 10.05 0.05 1,601 512 73 23 1 40 40.00 1,575 499 78

[0092] These results show that each negative electrode containing atleast one of the carbon fibers and the carbon flakes exhibits a highercapacity at a constant current of 5 A and a smaller internal resistancethan those of the negative electrode not containing the carbon fibersand the carbon flakes shown in Example 17. In addition, the capacity ata constant current of 5 A and the internal resistance are furtherimproved in each negative electrode containing both the carbon fibes andthe carbon flakes compared to each negative electrode containing eitherthe carbon fibers or the carbon flakes.

[0093]FIG. 1 is a schematic view of a scanning electron microscopecross-section of a electrode. The carbon fibers and carbon flakes aredisposed in the interstices between the electrode active materialparticles. The synergistic effects of the improved retention of the tesolution by the carbon fibers and the improved conductivity between theactive material particles by the carbon flakes facilitatedoping/undoping of lithium in a high-load current mode and increase thecapacity of the battery in the high-load current mode.

[0094] The capacity at a 5 A discharge current mode increases with theincreased carbon fiber content as shown in Examples 1 to 8, but thecapacity does not significantly increase when the carbon fiber contentincreases to 6 percent by weight as shown Example 21. Also, the capacitydoes not significantly increase when the carbon fiber content decreasesto 0.01 percent by weight as shown in Example 20. Thus, the capacity issignificantly improved and the internal resistance is reduced when thecarbon fiber content is in a range of 0.02 percent by weight to 5percent by weight, and particularly 0.5 percent by weight to 4 percentby weight.

[0095] As shown in FIG. 2, a carbon fiber content exceeding 5 percent byweight increases the internal resistance at 23° C., whereas a carbonfiber content of less than 0.02 percent by weight significantlyincreases the internal resistance at 23° C. Accordingly, it ispreferable that the carbon fiber content be in a range of 0.02 percentby weight to 5 percent by weight, as shown in range A in FIG. 2, andparticularly 0.5 percent by weight to 4 percent by weight, as shown inrange A′ in FIG. 2.

[0096] The capacity at a 5 A discharge current mode increases with theincreased carbon flake content as shown in Examples 1 and 9 to 16, butthe capacity does not significantly increase when the carbon flakecontent increases to 40 percent by weight as shown Example 23. Also, thecapacity does not significantly increase when the carbon flake contentdecreases to 0.05 percent by weight as shown in Example 22. Thus, thecapacity is significantly improved and the internal resistance isreduced when the carbon flake content is in a range of 0.1 percent byweight to 30 percent by weight, and particularly 1 percent by weight to10 percent by weight.

[0097] As shown in FIG. 3, a carbon flake content exceeding 30 percentby weight increases the internal resistance at 23° C. without asignificant increase in the capacity, whereas a carbon flake content ofless than 0.1 percent by weight significantly results in significantdeterioration of these characteristics. Accordingly, it is preferablethat the carbon flake content be in a range of 0.1 percent by weight to30 percent by weight, as shown in range B in FIG. 3, and particularly 1percent by weight to 10 percent by weight, as shown in range B′ in FIG.3.

[0098]FIG. 4 is a graph of the relationship between the capacity at a 5A discharge current mode and the ratio by weight of the carbon flakes tothe carbon fibers. It is preferable that the ratio be in a range of 0.2to 100, as shown in range C in FIG. 4, and particularly 0.4 to 20, ashown in range C′ in FIG. 4.

[0099] Obviously many modifications and variations of the presentinvention are possible in the light of the above description. Forexample, the sizes and types of the carbon fibers and the carbon flakescan be changed and combined without restriction. Moreover, fine carbongranules and other any additives may be added to the carbon fibers andthe carbon flakes.

1. A nonaqueous electrolyte secondary battery comprising: a positiveelectrode comprising a positive-electrode active material; a negativeelectrode comprising a particulate negative-electrode active material,said positive-electrode active material and said negative-electrodeactive material being capable of intercalating/deintercalating lithium;and a nonaqueous electrolyte solution; wherein said negative electrodefurther comprises carbon fibers and carbon flakes disposed in theinterstices between said particulate negative electrode active material;wherein the ratio by weight of said carbon fibers to said carbon flakesin said negative electrode is in a range of 0.2 to 100; wherein saidcarbon fibers are produced by vapor phase deposition; and wherein saidcarbon flakes have a (002) interplanar spacing of less than 0.3360 nm byX-ray diffractometry and a thickness of (002) c-axis crystallites of 100nm or more and the bulk density of said carbon flakes, as measured byJapanese Industrial Standard K-1469, is 0.4 g/cm³ or less, and themaximum particle size of said carbon flakes, as measured by laserdiffraction, is 50 μm or less.
 2. A nonaqueous electrolyte secondarybattery according to claim 1, wherein the content of said carbon fibersin said negative electrode is in a range of 0.02 percent by weight to 5percent by weight.
 3. A nonaqueous electrolyte secondary batteryaccording to claim 1, wherein the content of said carbon flakes in saidnegative electrode is in a range of 0.1 percent by weight to 30 percentby weight.
 4. A nonaqueous electrolyte secondary battery according toclaim 1, wherein said carbon fibers have an average diameter of 0.01 to1 μm and an average length of 1 to 100 μm.
 5. A nonaqueous electrolytesecondary battery according to claim 1, wherein said positive-electrodeactive material comprises a Li compound oxide and saidnegative-electrode active material comprises one of a Li compound oxide,an amorphous metal oxide, and a carbonaceous material.
 6. A nonaqueouselectrolyte secondary battery according to claim 5, wherein the Licompound oxide of said positive-electrode active material isLiM_(x)O_(y) wherein M is at least one selected from the groupconsisting of Co, Ni, Mn, Fe, Al, V, and Ti, and said negative-electrodeactive material comprises a carbonaceous material.
 7. A nonaqueouselectrolyte secondary battery according to claim 1, wherein saidnonaqueous electrolyte solution comprises at least one nonaqueoussolvent selected from the group consisting of propylene carbonate,ethylene carbonate, vinylene carbonate, dimethyl carbonate, diethylcarbonate, methyl ethyl carbonate, 1,3-dioxolane,4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane,acetonitrile, and propionitrile.
 8. A nonaqueous electrolyte secondarybattery according to claim 7, wherein said nonaqueous electrolytesolution comprises a nonaqueous solvent mixture of ethylene carbonateand dimethyl carbonate.
 9. A nonaqueous electrolyte secondary batteryaccording to claim 8, wherein said nonaqueous electrolyte solutioncomprises at least one electrolyte selected from the group consisting ofLiClO₄, LiPF₆, LiBF₄, LiB(C₆H₅)₄, LiCl, LiBr, CH₃SO₃Li, and CF₃SO₃Li.10. A nonaqueous electrolyte secondary battery according to claim 9,wherein said electrolyte solution comprises LiPF₆.
 11. A nonaqueouselectrolyte secondary battery according to claim 1, wherein said carbonfibers are graphitized at a constant temperature of at least 2000° C.12. A nonaqueous electrolyte secondary battery comprising: a positiveelectrode comprising a positive-electrode active material; a negativeelectrode comprising a particulate negative-electrode active material, aquantity of carbon fibers, and a quantity of carbon flakes wherein saidfibers and said flakes are disposed in the interstices between saidparticulate negative-electrode active material; and a nonaqueouselectrolyte solution; wherein said positive electrode and said negativeelectrode are wound together with a separator disposed therebetween;wherein the ratio by weight of said fibers to said flakes is in a rangeof from about 0.2 to 100; wherein said fibers are produced by vaporphase deposition; and wherein said carbon flakes have a (002)interplanar spacing of less than 0.3360 nm by X-ray diffractometry and athickness of (002) c-axis crystallites of 100 nm or more and the bulkdensity of said carbon flakes, as measured by Japanese IndustrialStandard K-1469, is 0.4 g/cm³ or less, and the maximum particle size ofsaid carbon flakes, as measured by laser diffraction, is 50 μm or less.13. A nonaqueous electrolyte secondary battery according to claim 12,wherein the content of said fibers is in a range of from about 0.02% byweight to 5% by weight.
 14. A nonaqueous electrolyte secondary batteryaccording to claim 12, wherein the content of said flakes is in a rangeof from about 0.1% by weight to 30% by weight.
 15. A nonaqueouselectrolyte secondary battery according to claim 12, wherein said fibershave an average diameter of from about 0.01 to 1 μm and an averagelength of from about 1 to 100 μm.
 16. A nonaqueous electrolyte secondarybattery according to claim 12, wherein said flakes have an averagediameter of from about 0.5 to 50 μm and an average thickness of fromabout 0.01 to 1 μm.
 17. A nonaqueous electrolyte secondary batteryaccording to claim 12, wherein said positive-electrode active materialcomprises a Li compound oxide and said negative-electrode activematerial comprises a compound selected from the group consisting of a Licompound oxide, an amorphous metal oxide, a carbonaceous material, andmixtures thereof.
 18. A nonaqueous electrolyte secondary batteryaccording to claim 17, wherein said negative-electrode active materialcomprises a carbonaceous material and said positive-electrode activematerial comprises LiM_(x)O_(y) wherein M is selected from the groupconsisting of Co, Ni, Mn, Fe, Al, V, Ti, and mixtures thereof.
 19. Anonaqueous electrolyte secondary battery according to claim 18, whereinsaid carbonaceous material is a graphite material.
 20. A nonaqueouselectrolyte secondary battery according to claim 17, wherein saidpositive electrode and said negative electrode further separatelycomprise a binder selected from the group consisting of a polyvinylidenefluoride, a polytetrafluoroethylene, an ethylene-propylene-dienecopolymer, a styrene-butadiene rubber, and mixtures thereof.
 21. Anonaqueous electrolyte secondary battery according to claim 20, whereinsaid binder is polyvinylidene fluoride.
 22. A nonaqueous electrolytesecondary battery according to claim 12, wherein said nonaqueouselectrolyte solution comprises at least one nonaqueous solvent selectedfrom the group consisting of propylene carbonate, ethylene carbonate,vinylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethylcarbonate, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether,sulfolane, methylsulfolane, acetonitrile, propionitrile, and mixturesthereof.
 23. A nonaqueous electrolyte secondary battery according toclaim 22, wherein said nonaqueous electrolyte solution comprises anonaqueous solvent mixture of ethylene carbonate and dimethyl carbonate.24. A nonaqueous electrolyte secondary battery according to claim 22,wherein said nonaqueous electrolyte solution comprises at least oneelectrolyte selected from the group consisting of LiClO₄, LiPF₆, LiBF₄,LiB(C₆H₅)₄, LiCl, LiBr, CH₃SO₃Li, CF₃SO₃Li, and mixtures thereof.
 25. Anonaqueous electrolyte secondary battery according to claim 24, whereinsaid electrolyte solution comprises LiPF₆.
 26. A nonaqueous electrolytesecondary battery according to claim 12, wherein said carbon fibers aregraphitized at a constant temperature of at least 2000° C.